In seawater desalination, ultrapure water production, industrial water treatment, wastewater reclamation, and the like, reverse osmosis membranes have been used for removing ions, organic substances, and the like included in raw water (see, non-patent literature 1). The permeation flux of a reverse osmosis membrane may be reduced by microorganisms that proliferate on the surface of the membrane and organic substances adsorbed on the surface of the membrane. Furthermore, a reverse osmosis membrane may be clogged with solid particles. This increases the pressure difference across a module. Accordingly, a reverse osmosis membrane needs to be cleaned periodically in order to restore the permeation flux of the membrane and the difference in pressure between the feed-side end and the concentrate-side end portion of an element (hereinafter, referred to as “pressure difference across an element”).
Some reverse-osmosis-membrane devices include an element having a membrane structure referred to as “spiral structure”. An example of the spiral membrane elements used in the related art is a spiral membrane element produced by bonding two reverse osmosis membranes disposed on respective surfaces of a permeate spacer along three sides thereof in order to form a bag-like membrane, attaching an opening portion of the bag-like membrane to a permeate collection tube, and subsequently winding the bag-like membrane around the outer periphery of the permeate collection tube with a net-like feed spacer in a spiral form. The raw water fed from an end portion of the element flows along the feed spacer and is discharged from the other end portion as a concentrate. While the raw water flows along the feed spacer, it passes through the reverse osmosis membrane as a permeate. The permeate flows into the permeate collection tube along the permeate spacer and is discharged from an end portion of the permeate collection tube.
In the spiral membrane element, the feed spacer, which is interposed between the bag-like membranes wound around the permeate collection tube, forms a raw-water path. Therefore, increasing the thickness of the feed spacer included in the spiral membrane element reduces the likelihood of the raw-water channel being clogged with solid particles and prevents an increase in the pressure difference across the element, a reduction in the amount of permeate, and the degradation of the quality of the permeate which may result from the accumulation of the solid particles. Spiral reverse-osmosis-membrane elements that include a feed spacer having a large thickness in order to reduce the occurrence of clogging with solid particles are on the market.
However, increasing the thickness of the feed spacer reduces the area of the membrane per element and the amount of permeate per element. The area of the membrane included in a commercially available spiral reverse-osmosis-membrane element is 42 m2 (440 ft2) or less.
Moreover, increasing the thickness of the feed spacer does not cause a reduction in the permeation flux which results from the adsorption of membrane foulants onto the membrane. Reducing the thickness of the feed spacer in order to increase the area of the membrane per element increases the risk of the channel being clogged with solid particles.
Organic wastewater, such as sewage, may be treated by a membrane-separation activated sludge process in which a membrane bioreactor (MBR) is used. The wastewater is subjected to an activated sludge process in a biological treatment tank. A liquid including activated sludge is subjected to solid-liquid separation in an immersed membrane-separation device, which is arranged in the biological treatment tank to be immersed in the liquid. non-patent literature 2 discloses a method for treating organic wastewater in which water treated with an MBR (permeate obtained by filtration through the filter included in the immersed membrane-separation device) is directly fed into a reverse-osmosis-membrane device and subjected to reverse-osmosis-membrane separation.
The MBR-treated water includes a large amount of high-molecular organic substance having a molecular weight of 10,000 or more, which acts as a membrane foulant. Accordingly, in a reverse-osmosis-membrane device that treats the MBR-treated water, the permeation flux may decrease with time, and the difference in pressure across the membrane may be increased.